It's hard enough for IT professionals to maintain the security of general purpose computing devices, let alone non-technical users achieving the same goal.
Lets take a real-world exmaple. Look at the incidence of malware on a platform that allows unrestricted software installation (e.g. Android), with one that doesn't (e.g. iOS).
The problem is that making informed choices about whether a given piece of software is "trustworthy" is extremely difficult. Delegating that to a 3rd party (e.g. Apple) is for most people, likely the best option.
We managed to somehow survive 30 years of unprecedented innovation and computer usage and NOW we suddenly need locked down devices where a single company decides which applications may be used on them and can forbid and innovative competition at will?
Why would you defend that?
OSX and Linux are both vulnerable to malware and have incidences of it, however (for desktop) they're relatively small targets, so get less attention. The idea that they're somehow resistent to malware is amusing.
and yep times are changing. I started in IT in 1995 and the environment now is massively more hostile than it was then, also the number of users has gone up a lot too, so there are a lot more people who aren't well placed to secure general purpose computing devices.
I'd argue that there's plenty of competition and if someone wants to use a nice open computing device, they can. The fact that most people choose not to, should tell you something about their priorities.
That's a common fallacy perpetuated by Apple and similar companies but it's simply not true. Having the ability to unlock your device, install apps after you enable unknown sources and hange other parts of it do not have to come at the cost for "average user" (whoever that is). You can make a secure simple default and still not waste development time and resources locking down the devices.